ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-9-1407-2009Intensification of tropical cyclones in the GFS modelMarínJ. C.1RaymondD. J.2RagaG. B.11Centro de Ciencias de la Atmósfera, UNAM, Mexico City, Mexico2Physics Department, New Mexico Tech., Socorro, USA200220099414071417This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/9/1407/2009/acp-9-1407-2009.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/1407/2009/acp-9-1407-2009.pdf

Special forecasts from the Global Forecast System (GFS) model were used in
this study to evaluate how the intensification process in a tropical cyclone
is represented in this model. Several tropical cyclones that developed in
2005 were analyzed in terms of the storm-scale circulation rather than more
traditional measures such as maximum wind or minimum central pressure. The
primary balance governing the circulation in the planetary boundary layer is
between the convergence of environmental vorticity, which tends to spin up
the storm, and surface friction, which tends to spin it down. In addition, we
employ recently developed ideas about the relationship between precipitation
and the saturation fraction of the environment to understand the factors
controlling mass, and hence vorticity convergence. The budget of moist
entropy is central to this analysis.
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Two well-known governing factors for cyclone intensification emerge from this
study; surface moist entropy fluxes, dependent in the model on sea surface
temperature and cyclone-generated surface winds, and ventilation of the
system by dry environmental air. Quantitative expressions for the role of
these factors in cyclone intensification are presented in this paper.